1. Field of the Invention
This invention relates to in situ recovery of shale oil, and more particularly to a method for using different blasting techniques in differing grades of oil shale when forming an in situ oil shale retort.
2. Description of the Prior Art
The presence of large deposits of oil shale in the semi-arid, high plateau western region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; 4,118,071; and 4,192,554; which are owned by the assignee of this application and incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded toward one or more void spaces excavated in the formation for forming a fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort.
During processing of a fragmented mass, retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted shale oil. One method of supplying hot retorting gases used for converting kerogen contained in oil shale includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting" . Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
When forming a fragmented mass, it is important that formation within the retort site be fragmented and displaced, rather than simply fractured to form a fragmented mass of reasonably high permeability; otherwise, too much pressure differential is required to pass a retorting gas through the fragmented mass. The particular techinques used for explosively expanding formation toward one or more void spaces within a retort site can affect the permeability of the fragmented mass. Bypassing portions of the fragmented mass by retorting gas can be avoided, for example, in a fragmented mass having reasonably uniform permeability in horizontal planes, section-by-section vertically, across the fragmented mass. Undesirable gas channeling through the fragmented mass can occur if there is non-uniform permeability.
Generally, when forming a fragmented mass, an array of blast holes are drilled in the zone of formation adjacent the void spaces excavated within the retort site. Explosive charges are placed in the blast holes and are detonated in a desired time delay sequence for explosively expanding formation toward the void spaces for forming the fragmented mass. It is important to develop a blasting technique that produces reasonably uniform particle size and void fraction in the fragmented mass for enhancing uniformity of permeability. It is also desirable to provide a blasting technique that makes efficient use of explosive energy. In this way, a fragmented mass with reasonably high and reasonably uniform permeability throughout the retort site can be achieved.
In situ retorts are commonly formed in formation having different average grades of oil shale section-by-section vertically through the retort site. The present invention is based on a recognition that the most effective use of explosive energy produced by an array of explosive charges is a function of the grade of oil shale within the retort site.
Oil shale deposits in the western United States occur in generally horizontal beds and within a given bed there are an extremely large number of generally horizontal deposition layers known as "varves". The kerogen content of the formation is typically nonuniformly dispersed throughout a given bed as the kerogen content varies from layer to layer.
The average kerogen content of formation containing oil shale can be determined by a standard "Fischer assay" in which a core sample customarily weighing 100 grams and representing one foot of core is subjected to controlled laboratory analysis involving grinding the sample into small particles which are placed in a sealed vessel and subjected to heat at a known rate of temperature rise to measure the kerogen content of the core sample. Kerogen content is usually stated in units of "gallons per ton", referring to the number of gallons of shale oil recoverable from a ton of oil shale heated in the same manner as in the Fischer analysis.
The average kerogen content of formation containing oil shale varies over a broad range from essentially barren shale having no kerogen content up to kerogen content of about 70 gallons per ton. Localized regions can have even higher kerogen contents, but these are not common. It is often considered uneconomical to retort formation containing oil shale having an average kerogen content of less than about 8 to 10 gallons per ton.
Formation containing oil shale which is suitable for in situ retorting can be hundreds of feet thick. Often, there are strata of substantial thickness within such formation having significantly different kerogen contents than other strata in the same formation. Thus, for example, in one formation containing oil shale in Colorado that is a few hundred feet thick, the average kerogen content is in the order of about 17 gallons per ton. Within this formation there are strata 10 feet or so thick in which the kerogen content is in excess of 30 gallons per ton. In another portion of the same formation, there is a stratum almost 30 feet thick having nearly zero kerogen content. Similar stratification of kerogen content occurs in many formations containing oil shale.
It has been found that greater energy and better distribution of explosive are needed to obtain a given particle size distribution and void fraction when rich oil shale is explosively expanded than are required to obtain similar particle size distribution and void fraction when lean oil shale is explosively expanded. Thus, it is desirable to develop a blasting technique for forming a fragmented mass that ensures effective use of explosive energy throughout a retort site having regions of various grades of oil shale. SUMMARY OF THE INVENTION
Briefly, an in situ oil shale retort is formed within a retort site in a subterranean formation containing oil shale and having a region of relatively richer oil shale and a region of relatively leaner oil shale at different elevations within the retort site. At least one void space is excavated within the retort site, leaving a remaining portion of formation including the regions of richer and leaner oil shale adjacent such void space. An array of explosive charges placed in the region of richer oil shale has a shallower scaled depth of burial (sdob) than an array of explosive charges placed in the region of leaner oil shale. This provides greater explosive energy collectively among the explosive charges in the region of richer oil shale when compared with the explosive energy collectively provided by the explosive charges in the region of leaner oil shale.
In one form of the invention, greater spacing between explosive charges in the region of leaner oil shale is used when compared with the spacing between explosive charges in the region of richer oil shale. This provides the shallower sdob for the explosive charges in the region of richer oil when compared with the sdob of explosive charges in the region of leaner oil shale, as well as improving distribution of explosive. According to one technique for providing such a spacing difference among explosive charges, blast holes are drilled on a pattern which, in effect, comprises two superimposed arrays of blast holes. Explosive charges in one array are loaded for a larger sdob in the region of leaner oil shale and both arrays are loaded for a smaller sdob in the region of richer oil shale. The explosive charges in both arrays are detonated for explosively expanding both regions of formation toward void space within the retort site for forming a fragmented mass. The greater explosive energy and smaller spacing distance within the region of richer oil shale, compared with the explosive energy and spacing distance within the region of leaner oil shale, can produce comparable results in terms of fragmentation when forming the fragmented mass.